1) Field of the Invention
The present invention relates to a color image forming apparatus for creating full-color images for use in an electrostatic-type copying machine, printer, facsimile device and the like. Particularly, the present invention relates to a color image forming apparatus that creates a color image on a recording medium by super imposing full-color toner images created by full-color image processing sections arranged along a conveyer belt.
2) Description of the Related Art
There have been color image forming apparatuses that create color images on a recording medium by superimposing a full-color toner image created by full-color image processing sections arranged along a conveyer belt. A typical example of such an apparatus is a full-color image forming apparatus of the tandem type.
In the color image forming apparatus of the tandem type, the alignment of the respective colors is important. The factors that affect positional deviation of colors are skew, positional deviation of the resist in the sub-scanning direction and the main scanning direction, unevenness of pitch in the sub-scanning direction, and magnification rate error in the main scanning direction. There have been many inventions made previously to correct the positional deviation of colors. For example, a technology for detecting the positions of a plurality of markers, also called as compensation patterns, formed on the conveyer belt is described in Japanese Patent Laid-Open Publication No. H08-50385 (Patent No. 2858735). This publication discloses a technology that automatically corrects the positional deviation of the basic colors with respect to the other colors in the main scanning direction and sub-scanning direction.
Further, a technology for automatically correcting the positional deviation of colors in the main scanning direction is described in Japanese Patent Laid-Open Publication No. S63-286864 (Patent No. 2642351). It has been disclosed in this publication to carry out the automatic correction by forming transverse lines or compensation pattern on the conveyer belt in the main scanning direction. This is followed by calculating the positional deviation of the oblique lines in the main scanning direction by comparing the ideal gap between the reference line and the oblique line with the actual detected gap. The main scanning write clock or read clock is then corrected based on the result. The color image forming apparatus of the tandem type detects the compensation pattern formed on the conveyer belt and automatically corrects the positional deviation based on the result of detection of the compensation patterns. However, the automatic correction process does not completely eliminate the positional deviation of all the colors. This is because the detection of the compensation pattern is not always accurate.
It is an object of this invention to provide a color image forming apparatus that can accurately match the position of each color when forming a color image on a recording medium.
The image forming apparatus according to one aspect of the present invention comprises a plurality of full-color image processing sections arranged along a conveyer belt, each of the image processing sections forming a toner image of a corresponding color on a recording medium in a superimposed fashion; a driver that drives the image processing sections; a compensation pattern forming unit that forms at least one compensation pattern on the conveyer belt; at least one sensor that detects the compensation pattern and outputs a signal when the compensation pattern is detected; a filter that filters out frequency components from the signal output by the sensor that do not correspond to frequency component of the compensation pattern, and outputs the remaining frequency component; a compensation pattern position recognizer that recognizes the position of the compensation pattern on the conveyer belt based on the output of the filter; a correction value calculator that calculates a correction value based on position of the compensation pattern recognized by the compensation pattern position recognizer; and a drive controller that controls the driver based on the correction value when forming a color image on the recording medium.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.